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LONDON, England (CNN) -- As Friday dawns in London, investigators are picking through the carnage created by a coordinated bomb attack on three of the city's Underground trains and a double-decker bus.

Officials are urging commuters to stay home Friday.

Prime Minister Tony Blair blamed Islamic extremists for the Thursday morning bombings and vowed to bring those responsible to justice.

In a televised address to his nation, he praised "the stoicism and resilience of the people of London" in the face of the bloodshed.

"We will not be intimidated," Blair said.

The Union Jack flew at half-staff over Buckingham Palace on Thursday evening after the British capital's bloodiest day since World War II.

Queen Elizabeth II will visit with some of those "affected by the tragedy" Friday, Buckingham Palace announced.

Home Secretary Charles Clarke, the Cabinet official in charge of domestic security, told the British television network ITN that police are facing "total carnage" at the bomb scenes.

After meeting with the government's Emergency Committee, Blair vowed "the most intense police and security service action to make sure that we bring those responsible to justice." (Full story)

Police do not know whether suicide bombers carried out the attacks or whether bombs had been left in packages on the Underground or in buses, according to Brian Paddick, Metropolitan Police deputy assistant commissioner.

Paddick said it wasn't clear whether the bombs were on the trains or in the tunnels.

U.S. law enforcement sources said investigators have discovered remnants of timing devices that may have been used in the train explosions -- but that no such fragments have been found at the site of the bus blast.

Investigators' first priority is to examine forensic evidence and video footage from the transport system's extensive surveillance system, Paddick said.

"We've had considerable success in the past using closed-circuit television footage in order to trace the movements of the people involved," he said.

The Group of al Qaeda of Jihad Organization in Europe claimed responsibility in a Web site posting. The authenticity of the claim could not immediately be verified. (Full story)

Blair stressed that the bombers did not reflect the views of most Muslims. (Transcript)

"We know that these people act in the name of Islam, but we also know that the vast and overwhelming majority of Muslims here and abroad are decent, law abiding people," he said.

London Mayor Ken Livingstone said the blasts were "mass murder" carried out by terrorists bent on "indiscriminate ... slaughter." He spoke from Singapore, where an announcement that London will host the 2012 Olympic Games inspired celebrations back home a day earlier. (Full story)

'Trapped like sardines'

Three blasts took place in the city's subway system and one more hit a double-decker bus, all within an hour at the height of morning rush hour. (Timeline)

Paddick said there were 700 to 900 people on each of the affected trains at the time.

Witnesses described the horror of seeing victims dying and with serious injuries. There were scenes of panic as power failed on crowded underground trains, and tunnels filled with smoke.

"We were all trapped like sardines waiting to die," said Angelo Power. "I honestly thought I was going to die, as did everyone else."

Police cordoned off areas around six stations in and around the city's center and financial area and brought in sniffer dogs to check the areas. Telephone traffic -- particularly by cell phone -- was nearly impossible.

One man, with blood streaming down the left side of his face from a wound on his temple, said he didn't "want to live through it again."

"There was a very loud bang, the lights went out, the carriage filled with smoke," he said. "We were all thrown forward."

"I was in the front carriage and people were severely injured there," he said, dispassionately, adding that his train had been in the tunnel between Kings Cross and Russell Square.

"Some people were very calm, others very panicky."

Jarvis Medhurst said he was working at the Tavistock Hotel when the bus exploded "literally 40 meters away."

"There was a massive explosion and a cloud of smoke, and then when the smoke started to die down, you could see the wrecked bus, which was on fire," he said.

"There were bodies everywhere. Heads and bits of bodies, heads and arms and legs all ripped away.

"There seemed to be kids lying around as well as adults. I'm just in shock, it's something I'll never forget."

After the explosions, transit authorities shut down the entire Underground system and bus service in the center of London.

Bus service was restored Thursday afternoon -- with increased security -- except in the vicinity of the explosions, said officials, who added they hope to have some Underground service restored on Friday.

Leaders react

Blair was in Gleneagles, Scotland, hosting the G8 leaders when he got word of the attack. (Full story)

"It's particularly barbaric that this has happened on a day when people are meeting to try to help the problems of poverty in Africa and the long-term problems with the environment," he said.

Just before leaving for London, Blair made a second statement, surrounded by the other leaders present at the conference. (World reaction)

"All of our countries have suffered from the impact of terrorism," he said.

"We are united in our resolve to confront and defeat this terrorism that is not an attack on one nation, but all nations and on civilized people everywhere." (Transcript)

He turned the summit over to Foreign Secretary Jack Straw while he traveled to London then returned late Thursday.

U.S. President Bush was among the somber leaders who stood behind Blair as he spoke. (Transcript)

"We will not yield to these people, will not yield to the terrorists," he said in a short statement after Blair departed. "We will find them; we will bring them to justice."

The U.S. State Department -- where the Union Jack flew at half-staff Thursday evening -- confirmed at least two Americans were injured in the bombings, and says another two may have been hurt.

The blasts also spurred U.S. authorities to boost security in major American cities, particularly those with passenger rail systems. (Full story)

At the United Nations, the Security Council condemned the bombings in a resolution that calls on all nations "to cooperate to bring these people to justice," British U.N. Ambassador Emyr Jones Parry said.